1. Field
The invention relates generally to drilling equipment for subterranean boreholes and more specifically to coring tools for obtaining core samples.
2. State of the Art
Coring comprises the drilling and removal of core samples of rock from a drilled borehole. This technique has long been used in the oil and gas drilling industry to obtain information about the rock being drilled. Good cores can provide data for production estimates, reserve calculations, and regarding the expected behavior of different subsurface formations during the drilling and producing periods. Especially, the porosity of the reservoir rock and its connate water saturation can be determined with high accuracy from core samples.
The procedure in coring is typically as follows. A coring bit cuts a cylindrical core, which slides into a non-rotating inner core barrel or tube. The core barrel is seated within an outer drill barrel which is attached to and rotates with the coring bit. The inner core barrel or tube includes a "catcher" for catching and retaining the core within the core barrel while the core barrel is being pulled from the drill hole.
To provide the most accurate and useful information, it is important that the core as removed from the core barrel be intact and that it accurately reflect the layers of rock from which it is drawn. The catcher plays an important role in maintaining the core in its virgin condition as it is drawn from the hole.
Most such catchers are designed for use in so-called consolidated formations, which are relatively solid, self-supporting, and generally hard. A core from such a formation will not fall apart easily and so a catcher need not cover the entire lower opening of the core barrel. In fact, a catcher can simply be a frictional fitting forming a narrowed portion of the core barrel through which the core cannot quite slide back and out of the core barrel.
However, when pulling cores from unconsolidated formations such as sandstone, catchers of this type are unsuitable. The core may crumble, pieces may be lost through the lower opening of the core barrel, and upper regions of the core may shift due to the loss of pieces. Thus, a catcher for use in unconsolidated formations must provide substantially complete closure of the cross-section of the core barrel. Also, in unconsolidated formations less force is required to break the core from the formation with which it is integral.
One type of core catcher designed for use in unconsolidated formations has a set of cooperating flapper valves which provide a relatively complete closure of the cross-section of the core barrel. However, the lips of the flappers can impede passage of the core into the barrel and cause jamming. Jamming of the core requires termination of the coring procedure.
Another type of core catcher, described in U.S. Pat. Nos. 4,605,075 and 4,606,416, includes an inner shroud or sleeve which conceals the flapper valves from the core until coring is terminated. When coring is complete, the sleeve is displaced to free the flappers to catch the core. Unfortunately, such devices are relatively complex and may occasionally fail, leading to partial or complete core loss. U.S. Pat. No. 4,607,710 describes a core catcher without an inner sleeve wherein the flapper valves are biased toward an open position and a collet release mechanism permits cams to drive the valves closed. This device is also relatively complicated and expensive to make.
Still another approach to a core catcher for unconsolidated formations employs a positive displacement mechanism which may be hydraulically or otherwise externally activated to extend the dogs into the core barrel (for example U.S. Pat. Nos. 4,651,835 and 4,552,229). This mechanism is also very complicated and expensive.
Because of their complexity, as mentioned above, the scavenging mechanisms are subject to failure, which can result in loss or disturbance of the core.
Accordingly, a need remains for a simple, inexpensive and reliable device to retain a core of an unconsolidated formation within the core barrel during its removal from the drill hole.